Is there any language which is known to have acquired verb inflectional morphology in the process of evolution?Languages like English have given up inflections on verb,but I havent heard about any which has acquired the PNG inflection on verb.Is there any?

Of course. There tends to be a bias (due to European languages at this time? due to obvious contrast?) for noticing "simplification" but of course the complexity must also come from somewhere.Usually this type of innovation would occur at an agglutinative stage in the language (or at least for they morphology in particular) the it would later develop into the inflectional forms you're probably referring to.

As for paradigms emerging, I haven't studied any specific examples but some have been documented, yes.

One specific example may be French: while the agreement forms have all but disappeared in the verbal paradigms, now some of the subject pronouns are beginning to cliticize to the verbs and become like agreement markers. For example, "je" (I) is not very optional now for verbs (for example, under coordination it's more common to repeat "je") and already the language is no longer pro-drop.

There has also been some research on the development of the paradigms in Arabic.

In the Uralic family, some members of the Samoyed branch, e.g. Nenets, are well-known for their highly complex verbal morphology. The current view is that these forms can't stem from Proto-Uralic and must be a later innovation. Of course, there is no attested evidence since there is a lack of early written tradition in much of the Uralic languages; the evidence comes from comparative analysis instead. For example, take a look at this grammatical sketch of Tundra Nenets by Tapani Salminen.

It seems that there are lot more evidence for languages giving up inflectional morphology than developing new. Does that imply language change involves simplification?

Well, people only really focus on the obvious simplifications. There's a lot of rebuilding and restructuring that happens. Just look at a random selection of PIE languages now, knowing that they developed into such divergent structures is direct proof that verbal morphology takes on all sorts of weird and wonderful twists and turns.

The process is called grammaticalisation and it's a new and rapidly growing field in linguistics. Basically, verbal endings usually start out as content words and then grammaticalise into structural words that live on the end of stems as grammatical endings.

Taking a quick route down the Italic branch of PIE just for an example, you can see in Latin the forms for the word 'have' eroded away and became the future tense in modern Italian today:

amare + habeo = ameròamare + habet = amerà

The original Latin for "I will love" was 'amabo' which going further back to PIE comes from 'am + bhwo' which is the root of the word plus the verb 'to be'. There is definitely a visible trend for some languages to move into more isolating/analytic morphology, which rely less on specific endings but have all sorts of other ways to play with tense ideas. The English form 'will' used to mean wish/want and then it grammaticalised and suddenly saying that you wish for something took on the meaning of intention and thus developed into a future marker today.

So, yes in some cases it involves simplification and in other cases it just involves altering the structure. In other cases it involves a rise in complexity. Some languages are definitely more expressive in their ability to easily use moods like the subjunctive to make fine-grained distinctions in interpretation of language which would sound clumsy, too-wordy and generally unnatural if the exact same semantics were to be attempted in a language without that easy-access to such possibilities.

Some languages seem to like going in one direction, but it's an incomplete pool of data to look at PIE languages and the trends visible there, and from that, deduce that language change involves simplification, when just out of the corner of the analysis window there is a language like Archi which can theoretically change a verb into over 1.5 million different forms. That kind of complexity isn't typical. Its complexity lies is language change.

To sum up the point, language change is exactly that - language change.

It seems that there are lot more evidence for languages giving up inflectional morphology than developing new. Does that imply language change involves simplification?

As I said above, this must be wrong, or we would all be consistently speaking simpler and simpler languages (to the point where we would just walk around saying "aaaaa" to each other for all meanings!). Languages are complex, so that much have come from somewhere. No, they do not always simplify.

I believe there are two reasons why it appears that way:1. Bias for European languages. Compared to classical Indo-European languages (Latin, Greek, Sanskrit, etc.) the modern languages are simple in the morphology. Therefore, if you assume that such a system is expected, the modern languages will look simpler. But other complexities do arise. For example, I'd say that V2 word order in German is quite complex, and that came about during the loss of some inflection.

2. It's easier to see things disappearing than note things appearing, based on the available data.But there is plenty of research of these forms appearing anyway. lx gave some examples above, and there is a lot of research in general into grammaticalization, a whole field that deals with exactly this sort of thing.